1. Field of the Invention
This invention relates in general to earth boring machines, and in particular to an improved wrench mechanism for making up and breaking out sections of pipe of a drill string.
2. Description of the Prior Art
One type of earth boring machine uses a powered rotary that is mounted to a carriage which moves up and down the mast. The top section of the drill string is connected to a threaded connection at the rotary. Slips are normally located at a work table for supporting the weight of the drill string when disconnected from the rotary. The slips are also used to prevent the drill string from rotating when another pipe section or the rotary is being connected or disconnected to the drill string.
When removing sections of pipe from the borehole, normally a first or top section of pipe is pulled up into the mast so that its connection with the second or next lower joint is exposed. The slips are then set to support the string and to prevent the string from rotating. If the rotary is then rotated in reverse to unscrew the first section, either the lower connection or the upper connection will break. If the lower connection breaks first, then the first section can be restrained with a wrench while the upper connection is broken. However, if the upper connection section breaks first, some means is required to transmit torque from the rotary to the first pipe section to break the lower connection.
One type of breakout mechanism, as shown in U.S. Pat. Nos. Re. 28,351 and 3,695,364, deliberately loosens the upper connection first. In the first patent, the upper connection is broken while the first section is still in the borehole and held by the slips. In the second patent, an attempt is made to loosen the upper connection first while out of the hole, but if the lower connection happens to loosen first, the first section must be lowered back into the hole so that it can be grasped by the slips. The drill string is then pulled up to expose the lower connection, while the upper connection at the rotary is still in the loosened condition. Then a wrench, connected to the rotary, slides down and locks the first section to the rotary by the use of keys inserted in flats in the drill pipe. The rotary is then rotated in reverse to breakout the lower connection.
One disadvantage of this arrangement is that when using very heavy strings such as in blind shaft drilling, damage to the loosened threads may occur during lifting. Also, very high pitch threads are being used now that allow complete uncoupling in approximately one full turn. Consequently a danger exists that the string may be dropped when lifting with loosened threads. Other solutions, as shown in U.S. Pat. No. 3,680,412, have a breakout mechanism that operates independent of the rotary and is positioned directly at the lower connection. However, additional means are required to restrain the upper section when the rotary is reversed to breakout the upper connection.
Another solution to the problem of removing sections of drill pipe is shown in U.S. Pat. No. 4,037,672. The device disclosed therein utilizes a control system with different torques for drilling, for make-up, and for breakout. During pipe removal, the upper connection is further tightened to a torque greater than the original make-up torque between the first and second section, to assure that the lower connection will break first. While successful, another approach rather than multiple torques is desired.
The system shown in U.S. Pat. No. 4,037,672 also discloses drill pipe having vertical slots for engagement by the gripping means as an improvement over flats such as shown in U.S. Pat. No. 3,460,638. If the pipe is not quite vertical with the mast, the vertical dogs of the wrenching means will not bear evenly against the sidewall of the slot. Uneven loading may cause breakage of a dog.